Trees get too tall to drink

August 11, 2008 6:17:45 PM PDT

WASHINGTON (AP) - August 11, 2008 --

The height of Douglas fir trees appears to be limited by their ability to raise water to the highest branches, a problem that can be appreciated by anyone who has struggled to suck a thick milkshake through a straw.

Sure, water is thinner than a milkshake, but some firs are trying to raise it up as much as 350 feet.

Moisture evaporating from leaves reduces pressure in the narrow channels inside the tree, drawing water upward.

But researchers say the process seems to have a height limit.

"People have always been fascinated by how some trees, such as Douglas fir or redwoods, can grow so tall," said Barb Lachenbruch, a professor of wood science at Oregon State University. "This is not an easy thing to do. Think about trying to drink water through a narrow, 350-foot-long straw. It takes a lot of suction."

The longer the column of water the more likely it is that an air bubble will get into the tube - a sort of tree embolism - blocking the water flow, researchers report in Tuesday's edition of Proceedings of the National Academy of Sciences.

At a point of about 350 feet, air bubbles become so common they defeat the tree's ability to move water upward, according to the researchers who are working to understand how trees adapt to their environment.

The research was led by Jean-Christophe Domec of North Carolina State University. It was funded by the U.S. Department of Agriculture.